Internal combustion engines emit a large amount of unburned hydrocarbons during cold engine start-up. In fact, a substantial fraction of the total emitted hydrocarbons have been found to occur during the first minutes due to the uncombusted hydrocarbons in the rich fuel mixture.
Low molecular weight hydrocarbons are especially troublesome as pollutants because they form ozone. Emission standards for low molecular weight hydrocarbons are becoming more stringent. Zeolites with relatively low SiO.sub.2 to Al.sub.2 O.sub.3 mole ratios are very suitable for adsorbing low molecular weight hydrocarbons. However, their thermal stablity is relatively low.
Release of hydrocarbons after start-up of an engine poses a special problem because at this point the temperature of the exhaust gas and the main catalyst are not high enough for conversion to innocuous products in the presence of conventional catalysts. The catalysts utilized in catalytic converter systems are generally ineffective at ambient temperature and must reach high temperatures, often in the range of 300.degree.-400.degree. C. before they are activated.
One approach to reducing cold start emissions is to temporarily adsorb hydrocarbons on zeolites. These methods involve using the zeolite adsorber in the exhaust stream in a "by-pass" mode. That is, the zeolite adsorber is exposed to the cold start exhaust, but once the main converter reaches its light-off temperature (about 300.degree.-400.degree. C.) a system of valves sends the exhaust directly to the main converter, by-passing the adsorber.
U.S. Pat. No. 4,985,210 relates to an exhaust gas purifying apparatus employing a 3-way catalyst which has either a Y-type zeolite or a mordenite as an adsorbent at the upstream side of the catalytic converter so that when the exhaust gas temperature is not higher than a specific temperature, a harmful component is adsorbed by means of the adsorbent, whereas when the exhaust gas temperature exceeds the specific temperature the harmful component is desorbed from the adsorbent and is introduced into the catalytic converter. Further, an activated carbon trapper and a by-pass are provided in parallel at the upstream side of the adsorbent so that the flow paths of the exhaust gas are selectively switched from one to the other in accordance with the level of the exhaust gas temperature.
U.S. Pat. No. 5,125,231 relates to an engine exhaust system designed to reduce hydrocarbon emissions having first and second catalytic converters. Exhaust gas is conveyed to the first and second converters depending on the temperature. Hydrocarbons are adsorbed in the second converter at lower temperatures, and desorb at higher temperatures and are conveyed to the first converter. A system of thermostatically controlled valves is used to convey the exhaust gas to the proper converter depending on the temperature.
In the above described by-pass modes of operation, the thermal demands on the zeolite in the adsorber are relatively simple. However, the demands on the valve system are quite significant, in that it must function reliably for at least 50,000 miles of driving, or even higher. In either situation, an expensive system of valves will be required, with safety and reliability always a concern.
Therefore, there is a need to develop an exhaust gas purification system which will effectively remove pollutants both at cold start and after warm-up, without the need for extra valving.